1. Field of the Invention
The present invention relates to a reproducing method, a reproducing apparatus, a recording and reproducing apparatus, and a magnetic recording medium, and particularly, to the reproducing method, the reproducing apparatus, the recording and reproducing apparatus, and the magnetic recording medium adequate for a high recording density of the magnetic recording medium.
2. Description of the Related Art
These years an improvement of a recording density in a magnetic recording medium such as a magnetic recording tape and a magnetic disk is remarkable. Therefore, corresponding to a high recording density of a recording medium, various technologies are suggested regarding a recording and reproducing apparatus and are in practical use. For example, various technologies are suggested for an adoption of an MR (Magneto Resistive) head as a recording/reproducing head, an improvement of an interface with the recording medium and the heads, and the like, and are in practical use.
Furthermore, also in a sphere of a signal processing technology, in order to recover a deterioration of an S/N ratio accompanied with the high recording density, a PRML system combining a PR (partial response) system and an ML (Maximum Likelihood) system is in practical use in a recording and reproducing apparatus for using a recording medium such as a magnetic disk, a digital VTR, a magnetic tape for a computer backup, and an optical disk.
As such the recording and reproducing apparatus, for example, there exists one disclosed in JP 2002-157827A. Meanwhile, U.S. Pat. No. 6,337,889 corresponding to JP 2002-157827A is hereby incorporated by reference hereinafter in its entirety.
In the PRML signal processing system are known various systems such as a PR4ML (PR (1, 0, −1)ML), EPR4ML (PR (1, 1, −1, −1)ML), EEPR4ML (PR (1, 2, 0, −2, −1)ML), EEEPR4ML (PR (1, 3, 2, −2, −2, −3, −1)ML), according to a number of terms of a signal sequence where an equalizing processing is performed.
Then according to a characteristic of a magnetic material used, if an optimum partial response signal processing is applied, it is effective because an information signal recorded in a magnetic recording medium with a high density can be restored with a low error rate. For example, a magnetic recording medium having a magnetic layer containing hexagonal ferrite as a magnetic material has a characteristic that a reproducing output is high and has a low noise in high density recording. However, because the hexagonal ferrite has a magnetization component of a vertical direction in an in-plane orientation or a random orientation, derived from a crystal structure thereof, an isolated reversion reproducing waveform becomes a unique waveform, where an isolated reversion reproducing waveform in the in-plane orientation and one in the vertical direction are added, and, for example, indicates an asymmetric waveform as shown in FIG. 2. To a specific isolated reversion reproducing waveform obtained in a case that such the hexagonal ferrite is used as the magnetic material, even if a conventional PRML signal processing optimized for a magnetic recording medium where an in-plane directional magnetization is recorded is applied, an optimum signal processing cannot be performed, and thus a specific characteristic of a hexagonal ferrite magnetic material that a reproducing output is high and has a low noise in high density recording cannot be utilized.
Consequently, it is strongly requested a reproducing method, a reproducing apparatus, and a magnetic recording medium optimized for a partial response signal processing system in reproducing an information signal from a high recording density of the magnetic recording medium.